1. Field of the Invention
The present invention is broadly concerned with magnetic induction heating systems and methods wherein an induction heatable object not physically connected to a magnetic induction heater can be heated and temperature regulated using Radio Frequency Identification (RFID) technology. More particularly, the invention is concerned with such systems, as well as the individual components thereof, wherein objects to be heated are equipped with RFID tags and the induction heaters include RFID readers; when a tagged object such as servingware is placed on a heater, the tag transmits information such as the class of object being heated, and the heater control circuitry uses the information to initiate and carry out an appropriate heating cycle for heating and temperature-regulating the object. In preferred forms, two-way transmissions between the tag and a reader/writer is established, with each having electronic memory to store relevant heating information. More precise temperature regulation is achieved using an RFID tag having an associated switch responsive to an external condition such as temperature experienced by the switch. The invention is applicable to virtually any type of induction heatable object such as food servingware.
2. Description of the Prior Art
U.S. Pat. Nos. 5,951,900 to Smrke, U.S. Pat. No. 4,587,406 to Andre, and U.S. Pat. No. 3,742,178 to Harnden, Jr. describe non-contact temperature regulation methods and devices employing magnetic induction heating. In these prior devices, radio frequency transmissions between an object to be heated and the induction appliance are employed in an attempt to control the induction heating process.
In Smrke, Andre, and Harnden a temperature sensor of some kind is attached to the object to be heated to provide feedback information which is transmitted to the induction appliance. In each case, aside from manual inputs by the user, changes to the power output from the induction appliance made by its controller are based solely upon information gathered and transmitted by the temperature sensor. Inasmuch as most objects to be temperature regulated are not homogeneous, this sole dependence upon feedback from the temperature sensor often leads to unwanted temperatures within certain portions of the object. For instance, when a sauce pan filled with dense food is placed upon an induction cooktop and the power is maintained at a constant level, the pan surface temperature quickly rises, whereas the food layer furthest away from the pan is still at ambient temperature. If a temperature sensor is placed upon the surface of the pan, the temperature measured at this point may have a unknown or variable relationship to the temperature of remote food layers. Thus, when the sensor reaches a pre-set temperature that the induction appliance's control unit attempts to maintain, much of the food may still be cold. Conversely, if the temperature sensor is placed adjacent the top layer of food, the pan surface may get excessively hot prior to this food layer reaching the desired temperature, resulting in scorched food near the pan surface.
Smrke attempts to solve this problem by requiring that the temperature sensor be placed upon the lid of a pot. Harnden teaches placing a temperature sensor in direct thermal contact with the ferromagnetic inner wall of a vessel. However, regardless of sensor location, the problems associated with heating a non-homogeneous object remain. Furthermore, neither proposed solution can prevent a temperature sensor from making imperfect thermal contact with its intended surface, a likely condition that leads to gross inaccuracies in temperature control. It is often difficult to manufacture a device so as to place one or more temperature sensors in perfect thermal contact. Also, over time, the thermal expansions and contractions that the sensor/object interface experience leads to imperfect thermal contact.
In addition to the requirement for a temperature sensor on or adjacent the object to be heated, the prior art devices also require periodic or continuous temperature measurement of the object, and thus periodic or continuous transmissions from the object to a receiver connected to the induction appliance. Neither Harnden, Andre, nor Smrke teach any practical means of preventing interference between these periodic or continuous RF transmissions and the main magnetic field produced by the induction appliance, so as to ensure proper receipt of feedback information.
In Harnden, a temperature sensor such as a thermistor provides a continuous variable voltage signal, corresponding to the temperature sensed, to a voltage control oscillator located within the object. The voltage control oscillator produces a variable frequency signal that corresponds to the sensed temperature. This variable radio frequency signal is transmitted to a receiving unit that is connected to the induction cooking range. In Andre, temperature measurements of the object are periodically transmitted to a receiving/controlling unit at constant intervals of time. Each temperature value is stored in the controlling unit's memory. A differentiating circuit then calculates the temperature difference and uses this information to control a heating element.
In order to the ensure proper reception of such temperature-based radio frequency feedback information, Harnden teaches that the output frequency of the feedback signal should be at least a megahertz or multiples thereof. This is not a practical solution for an emissions-regulated production appliance. In Andre and Smrke no consideration is given to any way of preventing interference between the RF temperature signal and the main magnetic field.
Furthermore, although temperature information from the object is important, it is often not sufficient to execute a proper heating operation to a desired regulation temperature within a desired period of time. For instance, it is well known that the power applied to an object placed upon an induction cooktop depends greatly upon the distance between the object's ferromagnetic material and the work coil of the cooktop. Should an object require a particular graduated power application to prevent overheating of some parts of the object while reaching the desired regulation temperature throughout the object, as in the earlier sauce pan example, it is essential that the proper power be coupled to the object during each graduation. Furthermore, most practical heating operations required that the prescribed regulation temperature be reached within a maximum prescribed time. This restraint makes it even more important that proper power be applied during each temperature gradation. A means to correct for inconsistent power coupling that is based upon comparisons between power measurements and stored power coupling data is essential to achieve consistent heating operations and accurate temperature regulation. Neither Smrke, Andre, nor Harnden address the transmission or use of other than temperature information.
Finally, although Smrke and Andre attempt to provide for multiple induction appliance operation with like-type objects, neither teaches how a single induction appliance may automatically differentiate between different types of objects placed upon it so as to apply a unique heating operation to each type. Andre employs differential temperature measurement to prevent overheating an object that is placed upon a different, unintended heating element. In Smrke, when more than one induction appliance exists, a central electronic unit that is connected to all induction appliances can accept signals from each transmitter attached to its respective pot and use them to determine which induction appliance the pot is atop. In neither case can a single induction appliance differentiate among various types of objects prior to commencement of heating of each object type.
RFID is an automatic identification technology similar in application to bar code technology, but uses radio frequency instead of optical signals. RFID systems can be either read-only or read/write. For a read-only system such as Motorola's OMR-705+ reader and IT-254E tag, an RFID system consists of two major components--a reader and a special "tag". The reader performs several functions, one of which is to produce a low-level radio frequency magnetic field, typically either at 125 kHz or at 13.56 MHz. The RF magnetic field emanates from the reader by means of a transmitting antenna, typically in the form of a coil. A reader may be sold in two separate parts: an RFID coupler, including a radio processing unit and a digital processing unit, and a detachable antenna. An RFID tag also contains an antenna, also typically in the form of a coil, and an integrated circuit (IC). Read/write systems permit two-way communication between the tag and reader/writer, and both of these components typically include electronic memory for the storing of received information.